In the field of structural analysis, the ability to determine the stresses which a structural body experiences can provide important feedback in the design and construction, as well as the integrity during the service life of such structural bodies. Typically, surface strain on the structural member can provide information regarding the stresses that the body is experiencing. This information can lead to the identification of stress concentrations, over-stressed areas, and general stress mapping for comparison to and calibration of predictive and/or analytical methods. Currently, a number of methods exist for measuring surface strain, including point and full-field methods.
Point methods include electrical resistance strain gauge methods and methods employing electro-optic sensors and optical methods. These methods typically require affixing a plurality of sensors at various locations on a structural body, or stepping the sensor across the structural body. When the structure is stressed each sensor or step indicates the surface strain at individual discrete points. In order to determine the strain over an entire body, numerous sensors must be located at critically stressed points on the surface or numerous iterative steps are required when movable sensors are used. Accordingly, these point methods can be cumbersome, making it difficult to determine the stresses over an entire surface of a structure.
A number of surface measurement techniques have been developed to overcome the limitations of the point methods, including brittle coatings, photoelastic coatings, Moiré, interferometric, thermoelastic and digital image correlation methods. Each of these methods can be useful for certain applications, but each have certain characteristics which limit their usefulness. Brittle coatings typically provide good qualitative information about the principal stress directions on objects. However, conventional brittle coating techniques can only test a part in one loading configuration and can only provide limited quantitative information. Moreover, methods for automated data collection using conventional coating techniques are not presently available.
Photoelastic coatings provide the shear stress and principal stress direction information on objects. Conventional photoelastic coating techniques are typically cumbersome and time consuming to apply to large bodies. Moiré methods are typically limited to flat objects and are not used on complex three-dimensional geometries. Interferometric methods, such as holographic interferometry, electric speckle pattern interferometry and shearography, require sophisticated vibration isolation which greatly reduces their applicability. Thermoelastic methods require cyclical loading of the specimens to generate surface temperature gradients related to the stress field. Digital image correlation methods can lack the sensitivity required to test parts in the material linear range in regions of high strain gradients.
U.S. Pat. No. 6,327,030 to Ifju et al. (Ifju '030), which shares several common inventors with the present application, discloses a strain-sensitive coating, a strain measurement system, and a method to determine strains on substrate materials. Ifju '030 is herein incorporated by reference in its entirety. The disclosed system can include a strain field mapping system which can be used to create a full-field strain map of the mechanical strain on a substrate material. The luminescent strain-sensitive coating is preferably a polymer-based coating, and can incorporate one or more luminescent compounds, such as luminescent dyes. Appropriate illumination stimulates the dye, which in turn emits a longer wavelength luminescence signal. Differences in excitation and emission wavelengths permit optical filtering of these signals. One or more characteristics of the luminescence emanating from the coating can then vary in relation to the strain on the substrate material. In one embodiment, the change in the morphology of cracks in the coating can cause variation in one or more characteristics of the luminescence emanating from the coating such that the strain on the substrate material can be determined by measuring the luminescence emanating from the coating.
U.S. Pat. No. 6,219,139 to Lesniak discloses a structural specimen coated with or constructed of photoelastic material. When illuminated with circularly polarized light the coated specimen will upon stressing, reflect or transmit elliptically polarized light, the direction of the axes of the ellipse and variation of the elliptically light from illuminating circular light will correspond to and indicate the direction and magnitude of the shear stresses for each illuminated point on the specimen. A preferred spray coating is formulated to produce a coating which is of a thickness such that only a quarter wave of birefringence is produced when the maximum stress is imposed on the coated specimen.
Dyes are disclosed by Lesniak, but only non-luminescent dyes. Specifically, non-luminescent dyes are disclosed exclusively for providing light attenuation data sufficient to solve for the thickness of the coating. Since the attenuation difference between two wavelengths of light are not sufficient to solve for coating thickness when all three main sources of amplitude variation are considered, that being coating thickness, the surface of coating reflection, and reflection from the surface of the specimen, three dyes are required to provide information to solve the three unknowns. By adding one or more non-luminescent dyes to a photoelastic coating so that the attenuation of the three colors red, green, blue are each substantially different and of a known amount, a RGB camera can be used to provide data sufficient to solve for the three main sources of amplitude variation.